interface/ftdi: remove "untested" warning from DP Busblaster

This is reported as working by mitsakos on IRC.

Change-Id: I26b97779c3e8e237c5b3996aa81183031b12ec0c
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1419
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>